In the course of practicing a wide variety of commercially important industrial processes, gaseous process streams (or more generally “gaseous systems”) are produced which are contaminated with pernicious quantities of mercury. The mercury contaminants have proved to be particularly difficult to remove or reduce to acceptable levels. One of the most pernicious forms of mercury pollution is finely aerosolized elemental mercury. This form of mercury is generated by coal-fired power generation and is present in natural gas. In the U.S. coal-fired power plants are the largest source of man-made mercury emissions to the air, accounting for approximately 40% of all mercury emissions. Under current circumstances, mercury is adsorbed on the aerosolized soot from coal burning. This soot eventually settles and the mercury adsorbed on the carbon is converted to methyl mercury, dimethyl mercury, and other forms, which accumulate in the food chain. Alternatively, techniques have been developed which will cause the carbonaceous soot to auto-ignite and convert to CO2 and H2O. When this occurs, finely aerosolized elemental mercury is produced. The mechanism for conversion of elemental mercury to methyl mercury and other forms is not well understood but is most certainly microbially mediated. It is estimated that 2000 tons of mercury is generated this way annually. Elemental mercury also occurs in natural gas in concentrations up to hundreds of micrograms per Nm3. This is a significant account considering that a typical plant will process millions of Nm3 per day.
Currently there is no technology that is considered optimal for remediation of the mercury in its elemental aerosolized form. Although coalescers, brominated adsorbents, and other methods have been used, they either lack effectiveness or have significant negative aspects such as generation of large amounts of mercury-polluted material to be land-filled. Coalescers lack effectiveness due to the extremely small size and high surface tension of the droplets and also due to the lack of affinity for mercury of typical coalescer materials. Also known is a process based on photochemical oxidation. This has chiefly been known for use in treating flue gas wherein ultraviolet (UV) light is introduced into the flue gas, to convert elemental mercury to an oxidized form (i.e. mercuric oxide, mercurous sulfate, and mercurous chloride). Once in the oxidized form, mercury can be collected in existing air pollution control devices such as wet SO2 scrubbers, electrostatic precipitators, and baghouses (fabric filters).
None of the foregoing techniques, however, have been fully successful in treating gaseous systems of the type with which the present invention is concerned. In addition to human and ecological effects, mercury in this elemental finely aerosolized form compromises the integrity of the steel and iron in the plants and pipeline for processing and transporting the gas, sometimes resulting in catastrophic failure and explosions or uncontrolled releases. It would be most desirable to capture and coalesce the droplets of mercury and to remove the mercury from these gaseous streams in its pure and elemental form, thus eliminating release and/or production of great quantities of mercury-polluted adsorbent.